Miocene

The Miocene epoch is a major division of the geologic timescale and the first epoch of the Neogene period of the Cenozoic era (or the fourth epoch of the Tertiary sub-era). The Miocene follows the Oligocene epoch and is followed by the Pliocene epoch.

The Miocene extends from about 23.03 to 5.332 million years before the present. As with other older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain. The Miocene boundaries are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Oligocene and the cooler Pliocene.

The Miocene was a time of great change and during this epoch the foundation was laid for the life, climate, and geography of the modern world (Smith 2006).

The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words μείων (meioon, less) and καινός (kainos, new) and means "less recent." It is "less recent" than the Pliocene and had 18 percent less modern sea invertebrates than the Pleiocene.

Cenozoic era (65-0 mya)
Paleogene	Neogene      Quaternary

Tertiary sub-era			Quaternary sub-era
Neogene period
Miocene		Pliocene	Pleistocene	Holocene
Aquitanian	Burdigalian	Zanclean	Early
Langhian	Serravallian	Piacenzian	Middle
Tortonian	Messinian	Gelasian	Late

Miocene subdivisions

There are several different systems used to establish the Miocene faunal stages (divisions devised from fossils).

International Commission on Stratigraphy. The faunal stages are typically named according to the International Commission on Stratigraphy. These subdivisions are defined by the relative abundance of different species of calcareous nanofossils (calcite platelets shed by brown single-celled algae) and foraminifera (single-celled protists with diagnostic shells). Two subdivisions each form the Early, Middle, and Late Miocene. These subdivisions, from youngest to oldest, are:

Messinian	(7.246 – 5.332 mya (million years ago))
Tortonian	(11.608 – 7.246 mya)
Serravallian	(13.65 – 11.608 mya)
Langhian	(15.97 – 13.65 mya)
Burdigalian	(20.43 – 15.97 mya)
Aquitanian	(23.03 – 20.43 mya)

North America: Land mammal fauna. In most of North America, faunal stages are defined according to the land mammal fauna (NALMA). They overlap the borders of the Miocene and Oligocene/Pliocene:

Hemphillian	(9 – 4.75 mya); includes much of the Early Pliocene
Clarendonian	(11.8 – 9 mya)
Barstovian	(15.5 – 11.8 mya)
Hemingfordian	(19 – 15.5 mya)
Arikareean	(30.5 – 19 mya); includes much of the Oligocene

California sites. Californian sites, which are derived from the former Farallon Plate, provide another sequence which also overlaps with the epoch boundaries:

Delmontian	(7.5 – 2.9 mya); includes much of the Pliocene
Mohnian	(13.5 – 7.5 mya)
Luisian	(15.5 – 13.5 mya)
Relizian	(16.5 – 15.5 mya)
Saucesian	(22 – 16.5 mya)
Zemorrian	(33.5 – 22 mya); includes nearly all the Oligocene

Other systems. Yet other systems are used to describe the Miocene stratigraphy of Japan, Australia, and New Zealand.

Miocene paleogeography

Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent.

Mountain building took place in Western North America and Europe. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in the American Great Plains and in Argentina. India continued to collide with Asia, creating more mountain ranges.

The Tethys Seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish-Arabian region between 19 and 12 mya. Subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis) near the end of the Miocene.

Miocene climate

Climates remained moderately warm, although the slow global cooling that eventually led to the Pleistocene glaciations continued.

Although a long-term cooling trend was well underway, there is evidence for a warm period during the Miocene when the global climate rivaled that of the Oligocene. The Miocene warming began 21 mya and continued until 14 mya, when global temperatures took a sharp drop. By eight mya, temperatures dropped sharply once again, and the Antarctic ice sheet was already approaching its present-day size and thickness. Greenland may have begun to have large glaciers as early as seven to eight mya, although the climate for the most part remained warm enough to support forests there well into the Pliocene.

The oceans continue to cool as the poles were transformed into glaciers.

Miocene biota

Miocene flora

Grasslands appear to have underwent a major expansion as forests fell victim to a generally cooler and drier climate overall. Grasses also diversified greatly into a number of species and also caused a major increase in the biodiversity of large herbivores and grazers, including ruminants (of which modern cattle and deer belong).

Miocene fauna

Both marine and continental fauna were fairly modern, although marine mammals were less numerous. Only in isolated South America and Australia did widely divergent fauna exist.

Mammals. These were also modern, with recognizable wolves, raccoons, horses, beaver, deer, camels, and whales. A plethora of Miocene hominoid (ape) fossils have been found in both Eurasia and Africa, with most of the Middle and Late Miocene hominoids discovered in Eurasia (Smith 2006). It is believed that during the Early and Middle Miocene, African hominoids first immigrated to Eurasia (Smith 2006). Two Miocene fossil hominoids, Dryopithecus and Ouranopithecus, are classified under the family Hominidae (hominids), a taxon that includes the great apes (gorillas, chimpanzees, bonobos, orangutans), as well as humans and extinct relatives of humans, such as Australopithecus (Smith 2006).

Birds. Recognizable crows, ducks, auks, grouses, and owls appear in the Miocene. By the epoch's end, all or almost all modern families are believed to have been present; the few post-Miocene bird fossils that cannot be placed in the evolutionary tree with full confidence are simply too badly preserved instead of too equivocal in character. Marine birds reached their highest diversity ever in the course of this epoch.

Sea life. Brown algae, called kelp, proliferate, supporting new species of sea life, including otters, fish, and various invertebrates. The cetaceans diversified and some modern genera have already appeared, such as the sperm whales. The pinnipeds, who appeared near the end of the Oligocene, are more and more aquatic.

See Also

• Paleobotany
• Paleoclimatology
• Geologic time scale

References

ISBN links support NWE through referral fees

• Begun, D. R. “Miocene fossil hominids and the chimp-human clade.” Science, 257(5078): 1929-1933, 1992.
• Malone, D. “Mechanisms of hominoid dispersal in Miocene East Africa.” Journal of Human Evolution 16(6): 469-481, 1987.
• Ogg, J. Overview of Global Boundary Stratotype Sections and Points (GSSP's) 2004. Retrieved September 17, 2007.
• Rohde, R. A. GeoWhen Database 2005. Retrieved September 17, 2007.
• Smith, J. European Miocene Hominoids: The Missing Link? 2006. Retrieved September 17, 2007.